A standard inverted L antenna is much like it sounds. The antenna runs along a vertical component that is connected at its base to a ground plane and a horizontal extension is connected to the top of the vertical component to form an inverted “L.” Typically, the inverted L antenna is constructed as a transmitting and/or receiving antenna for use with short wave radios, and the like. While these traditional inverted L antennas may be obscured along a tree line or a building, they are still quite visible.
It is understood that antenna performance is dependent upon the relationship between the antenna length and the wavelengths of operation. Generally, an antenna's mode is labeled as a fraction of a wavelength.
More recent antenna developments include meander line antenna couplers, used with vertical conductors attached to a ground plane, where the vertical conductors are bridged by a horizontal conductor. See, for example, U.S. Pat. Nos. 5,790,080 and 6,492,953, Applicant's own work. There, meander line antenna couplers consist of slow wave, meander lines in the form of folded transmission lines mounted on a plate. By varying the distance between the line and the base plate, sections of varying impedance can be created to form the slow wave structure.
SWR, or standing wave ratio, is a measure of the impedance matching of loads to the characteristic impedance of a transmission line. Impedance mismatches result in standing waves along the transmission line. SWR is defined as the ratio of the partial standing wave's amplitude at an antinode (maximum) to the standing wave's amplitude at a node (minimum) along the line. SWR is usually thought of in terms of the maximum and minimum AC voltages along the transmission line, thus called the voltage standing wave ratio, or VSWR. EIRP is the amount of power that a theoretical isotropic antenna (i.e., an antenna that evenly distributes power in all directions) would emit to produce a peak power density observed in the direction of maximum antenna gain. EIRP takes into account the losses in transmission line and connectors and includes the gain of the antenna along with the RF power available. The EIRP is stated in terms of decibels over a reference power emitted by an isotropic radiator with equivalent signal strength in a given direction.
Some disadvantages of previous antennas include difficulty in achieving a low voltage standing wave ratio (“VSWR”) thus reducing the efficiency of antenna and reducing its gain. For example, in a transmit system a reduced gain limits the antenna's ability to deliver the required effective isotropic radiated power (EIRP). Similarly, in a receive system a loss of gain lowers the receive sensitivity. Thus, in a transmit system the loss in gain due to mismatch losses associated with higher VSWR means greater RF power must be available to the antenna to achieve a desired EIRP. In a receiving system the loss in gain means the received signal level is weaker, even too weak to process.
The present disclosure provides antennas with improved gain with lower VSWR that deliver improved EIRP and/or receive sensitivity. In certain embodiments, the antennas are electronically tuned and are ideally suited for a space-constrained environment based, in part, on the co-planar relationship between the impedance sections.